Tactics trainer: add remaining motif sets (discovered attack, back-rank mate, deflection, decoy)

[Fazel94]

From TODO.md » Education & training » Tactics trainer (P1, highest training ROI).

Context

The /tactics/ page (course/docs/tactics.md) and puzzle_set widget
({{ puzzle_set(id, puzzles, title) }} → src-widgets/widgets/puzzle-set-widget.js)
already ship 3 of 7 planned motifs: fork «چنگال», pin «آچمزی», skewer «سیخ»
(2 puzzles each).

Task

Add puzzle sets for the remaining 4 motifs using the same pattern:

• Discovered attack
• Back-rank mate
• Deflection
• Decoy

Approach (per prior sessions' decision, see pgns/README.md)

Hand-author FEN positions per motif (not pulled from Lichess/Chessable),
independently verify each with python-chess (legal FEN + legal solution
moves), document sourcing in pgns/README.md » "Chapter-embedded puzzles".

Acceptance criteria

• 4 new {{ puzzle_set(...) }} sections added to course/docs/tactics.md,
  one per motif, 2 puzzles each (matching existing motifs' shape)
• Each FEN + UCI solution independently verified with python-chess
• Sourcing documented in pgns/README.md
• Glossary entries cross-link to the new sets where applicable (motif
  explainer + worked example is a separate, still-open follow-up item)